For severely multipath dispersive environments, the downlink capacity of cellular Code Division Multiple Access (CDMA) systems, such as IS95, is limited by intra-cell interference. Downlink signals in a cell are orthogonal or substantially orthogonal at the transmitter (hereinafter the term orthogonal or substantially orthogonal includes the terms orthogonal and/or substantially orthogonal), but tend to suffer a loss of orthogonality due to multi-path propagation, time jitter (imprecise implementation errors), etc. The uplink capacity of CDMA systems, such as IS95, is also limited because the uplink signals are not orthogonal. However, orthogonal signals may be used for the uplink if different user symbols are received with timing alignment.
For nondispersive channels, time alignment of the signals results in orthogonality and rejection of the intra-cell interference. In reality, however, wireless channels are dispersive and propagation along multiple paths causes time misalignment in which case multi-path components will interfere with other multi-path components having a different delay. In this case it is generally only possible to align a single component from each signal. Therefore, orthogonality is partially lost due to the interference between non-aligned terms.
Accordingly there exists a need for a system and method of spreading which is resistant to misalignment (delay) caused by multi-path delays.
There also exists a need for a system and method of spreading which is resistant to interference caused by time jitter of the system.
A need also exists for a system and method of spreading which is resistant to interference caused by misalignment of the signals received from different users.
Accordingly it is an object of the present invention to provide systems and methods of spreading in CDMA systems which are resistant to effects of misalignment caused by multi-path propagation delays.
It is another object of the present invention to provide systems and methods of generating spreading codes which are resistant to effects of time jitter of the system.
It is a further object of the present invention to provide systems and methods of generating spreading codes which are resistant to effects of misalignment of signals from different users.
It is still another object of the invention to provide such codes which do not affect the data rate of the system.
These and other objects of the invention will become apparent to those skilled in the art from the following description thereof.